Printers are used to print output from computers, or similar type of devices that generate information, onto a recording medium such as paper. Commonly available types of printers include impact printers, laser printers and ink jet printers. The term “ink jet” covers a variety of physical printing processes and hardware but basically transfers ink from an ink supply to the recording medium in a pattern of fine ink drops. Ink jet print heads produce drops either continuously or on demand. “Continuously” means that a continuous stream of ink drops is created, e.g. by pressurizing an ink supply. “On demand” differs from “continuous” in that ink drops are only generated on demand, by manipulation of a physical process to momentarily overcome surface tension forces that keep an ink in the meniscus of a nozzle. The nozzle is located in a boundary surface of a small ink chamber. The most common practice is to suddenly raise the pressure on the ink in the ink chamber, thereby breaking the meniscus and ejecting a drop of ink from the nozzle. One category of drop-on-demand ink jet print heads uses the physical phenomenon of electrostriction, a change in transducer dimension in response to an applied electric field. Electrostriction is strongest in piezoelectric materials and hence these print heads are referred to as piezoelectric print heads. The very small dimensional change of piezoelectric material is harnessed over a large area to generate a volume change that is large enough to squeeze out a drop of ink from the ink chamber. A piezoelectric print head may include a multitude of ink chambers, arranged in an array, each chamber having an individual nozzle and a percentage of transformable wall area to create the volume change required to eject an ink drop from the nozzle, in accordance with electrostriction principles. Another category of drop-on-demand ink jet print heads uses heater-resistors in the ink chambers. A short voltage pulse is applied to the heater-resistor, thereby warming up the ink in contact with the resistor sufficiently for the ink near the contact surface to boil. The local liquid-to-vapor transition results in a local volume expansion of the liquid. This local volume expansion generates a pressure pulse ejecting a drop of ink out of the nozzle. Most of the on-demand ink jet print heads are characterized by having elongated chambers and a nozzle at one end of these chambers. These devices are therefore often referred to as end-shooter devices.
A problem with such end-shooter devices is that during periods of non-use, the ink that is retained in the ink chambers may deteriorate and lead to sedimentation of solid particles from the ink in the chamber. Deterioration of the ink in the chamber may also include evaporation of VOC's (volatile organic compounds) contained in the ink, at the ink meniscus. This may lead to a change in viscosity of the ink in the vicinity of the nozzle, having a negative effect on its jetting properties. Sedimentation and evaporation of ink components may potentially lead to a nozzle fall out or nozzle blockage. Another problem often causing operating failure of the print head is the presence of air bubbles in the ink chamber of end-shooter print heads. All these effects reduce the reliability of end-shooter print heads.
Some of these problems are addressed in U.S. Pat. No. 5,155,498. In this patent specification the print head includes an additional purging channel in the actuator of the ink jet print head. This channel allows ink to be flushed through the ink chamber and through the purging channel during a purging operation. The solution enables an improved maintenance of end-shooter print heads by a dedicated design of the ink flow in the print head actuator. A disadvantage of the purging channel however is that the ink is only replenished periodically, i.e. only during the purging operations. European patent EP 1 200 266 suggests an alternative print head design. This patent provides a continuous flow of ink in the ink chamber by dividing the ink chamber in an input or supply compartment and an output or drain compartment. The ink may continuously flow from input to output, thereby also replenishing the ink near the nozzle. A disadvantage of the proposed solutions however is that they include modifications to the basic geometry and acoustic behavior and operating conditions of the end-shooter ink chambers in the print head, and that the applicability of the proposed solutions are strongly related to the piezo shear mode technology. In U.S. Pat. No. 5,818,485 a continuous ink path is established through a side shooter thermal ink jet print head by forming ink channels in various internal portions of the print head. The invention suffers from similar disadvantages than the invention disclosed in EP 1 200 266 in that it requires adaptations to the ink chamber.
It would therefore be advantageous to have a improved print head and a method for reliably ejecting drops of ink from an ink chamber, based on established and proven end-shooter type print head designs, and without changing these proven designs.